The present invention relates generally to imaging systems, and more particularly to a method and system for determining a material composition of an imaged object by computing an effective atomic number.
Computed tomography (CT) systems generate two-dimensional (and in some cases, three-dimensional) images representative of an object or subject. In general, CT images may be presented in slices (or volumes) that are reconstructed from many projection images made at different angular positions around the object or subject. In many medical and industrial applications, it is often useful to determine the composition of materials imaged by these techniques. Such material identification may assist in determining what objects or structures are present in the subject. In medical imaging, for example, the material composition may be indicative of different tissue structures, such as soft tissue, bone, pathologies, and so forth. In other fields, such as package, luggage, and part inspection, other structures or objects of interest may be similarly identified by reference to the materials of which they are comprised.
Both CT systems and conventional X-ray imaging modalities provide representations of objects under examination in terms of attenuation coefficients, that is, the degree to which X-rays are attenuated by intervening structures between the source and detector. In typical CT imaging systems, the images generated usually provide information about linear attenuation coefficients of structures of the object. Linear attenuation coefficients are found to be useful when materials in the object are of the same density but may not be sufficient to provide density information of a material. Thus, linear attenuation coefficients are usually inadequate to characterize the material comprising an object.
There is a continuing need for improved methods and systems to compute material compositions in X-ray, CT and similar systems, and particularly that will permit determining or estimating effective atomic numbers for materials imaged.